Many light hydrocarbon gas reserves are found in areas of the world that are remote to any markets for the light hydrocarbon gas. Such light hydrocarbon gas is referred to as natural gas. This natural gas may contain condensates that are light gasoline boiling range materials as well as C3-C5 gaseous hydrocarbons and methane.
Frequently such natural gas also contains undesirably high quantities of water, acid gas compounds such as sulfur compounds, carbon dioxide and the like for liquefaction to produce liquefied light hydrocarbon gases, which typically comprise primarily methane and which are frequently referred to as liquefied natural gas (LNG).
When such gases are liquefied, the capacity of the liquefaction plant is determined primarily by the available market for the gas, the availability of transportation to the market and the like. Accordingly in many instances it is desirable to increase the capacity of the liquefaction process in incremental stages as the market expands to remain in balance with the available market. Accordingly light hydrocarbon gas liquefaction processes, referred to herein as natural gas liquefaction processes or LNG processes, are typically installed in trains. The term “train” as used herein refers to a series of vessels capable of, pre-treating and liquefying natural gas. The gas is desirably treated to remove acid gases and water to very low levels prior to charging it to the liquefaction zone. The train also includes compression facilities for compressing the refrigerant required for the refrigeration vessel and the like. The train is an integrated process for producing a selected quantity of liquefied natural gas. Previously, industry has expanded plant capacity by adding one or more additional trains (each of which includes its own gas pretreatment equipment, liquefaction equipment, as well as liquefied product transport and storage facilities), as required to meet the available market demand and the like. Such trains have been previously designed to produce a certain quantity of liquefied product with no future expansion of the train having been considered in the design.
In FIG. 1, an embodiment of a light hydrocarbon gas liquefaction system and process (train) is schematically shown. The system and process, as shown, includes a refrigeration cryogenic heat exchanger 15. As shown, compressed refrigerant is supplied to cryogenic heat exchanger 15 by turbines 31, 33, 35, and 37, which are shaft coupled to refrigerant compressors 32, 34, 36, and 38, respectfully. High-pressure refrigerant is supplied to compressors 32 and 34 by high-pressure refrigerant lines 61 and 62. These lines typically return high-pressure refrigerant from cryogenic heat exchanger 15 after it has served its purpose as a refrigerant and has been warmed to a substantially gaseous condition. Compressed high-pressure refrigerant is supplied to cryogenic heat exchanger 15 via lines 63 and 64. Low-pressure refrigerant is supplied to compressors 36 and 38 by low-pressure refrigerant lines 65 and 66. These lines typically return low-pressure refrigerant from cryogenic heat exchanger 15 after it has served its purpose as a refrigerant and has been warmed to substantially gaseous condition. Compressed low-pressure refrigerant is supplied to cryogenic heat exchanger 15 via lines 67 and 68. No significance should be attributed to this except that refrigerants can be produced from compressors 32, 34, 36, and 38 at different pressures if desired and passed to cryogenic heat exchanger 15 at different points in the refrigeration process if desired. The same or different refrigerants can be used in the high-pressure and low-pressure refrigerant loops, as known to those skilled in the art.
Further, an inlet light hydrocarbon gas that has desirably been treated to remove acid gases and water is charged to cryogenic heat exchanger 15 via line 59. The liquefied light hydrocarbon gas product is produced through line 69. Typically, a natural gas or other light hydrocarbon gas stream is introduced to acid gas removal vessel 10 via line 40. Acid gas regenerator 11 is shown in fluid communication with acid gas removal vessel 10 via lines 41 and 42. The treated gas is typically recovered from vessel 10 through line 43. The recovered gases are passed via lines 44, 45, and 46 to designated dehydration vessels 20, 21, and 22. Typically vessel 10 is an aqueous amine scrubber, operating as known to those skilled in the art. The aqueous amine may be selected from materials such as digycolanolamine (DGA), diethylamine (DEA), methyldiethanolamine (MDEA), methylethylanolamine (MEA), SULFINOL (trademark of Shell Oil Company), activated methyldiethanolamine (aMDEA), and combinations thereof. Carbon dioxide is typically removed to levels less than about 60 parts per million (ppm) while sulfur is typically removed to levels less than about 4 ppm through vessels such as acid gas removal vessel 10.
The general operation of such acid gas removal vessels, as shown, is well known to those skilled in the art. Since each train has been typically constructed separately as market demands require, it is common to provide an acid gas removal vessel and an associated acid gas regenerator for each train. This has also been the case for other components of such trains and associated infrastructure.
Since the aqueous amine process produces a gas that is relatively saturated in water and since the water freezes at a temperature much higher than methane, which constitutes the majority of the natural gas stream to be liquefied, it is necessary that at least a major portion of the water be removed from the gas stream. Treated water-saturated gas is recovered from acid gas removal vessel 10 via line 43 where it is passed to dehydration vessels 20, 21, and 22 via lines 44, 45, and 46, respectfully. Water is selectively removed through dehydration vessels 20, 21, and 22 to produce a dewatered gas in lines 54, 55, and 56. The dehydrated gas from vessels 20, 21, and 22 is then combined and passed to cryogenic heat exchanger 15 via line 59. Typically, these dehydration vessels contain an adsorption material such as a molecular sieve, activated alumina, or the like. Such material is effective in removing the water from an inlet gaseous stream to extremely low levels, thus rendering the gaseous stream suitable for liquefaction in cryogenic heat exchanger 15. Typically three vessels are placed in each train to meet the requirements to dehydrate incoming gas. The process may also use adsorption materials for removal of other contaminants, such as mercury.
In the use of dehydration vessels 20, 21, and 22, two vessels will generally serve to remove the water from its associated feed gas stream, 44, 45, or 46, while one vessel is being regenerated by hot regeneration gas. Such configuration is depicted in FIG. 1 where dehydration vessels 20 and 21 serve to produce relatively water free gas streams 54 and 55 by removing water from inlet gas streams 44 and 45. Dehydration vessel 22, in the depicted configuration, is being regenerated by hot regeneration gas where the regeneration gas enters the vessel via line 70 and exits via line 71. All dehydration vessels 20, 21, and 22 all have the capability to operate in either dehydration or regeneration mode (though not shown for simplicity), as indicated in FIG. 1 by vessel 22 and process streams 70 and 71. Typically three vessels are placed in each train to meet the requirements of dehydration the incoming gas.
The acid gas removal vessels are readily regenerated as well known to those skilled in the art by a variety of techniques. One commonly used technique is the use of a reboiler on vessel 11 for regeneration.
A wide variety of refrigeration processes are contemplated within the scope of the present invention. No novelty is claimed with respect to the particular type of refrigeration process or vessel used. The process of the present invention is considered to be useful with any type of liquefaction process that requires light hydrocarbon gas as an inlet stream.
Clearly, the construction of separate trains of refrigeration processes as discussed above results in the expenditure of considerable capital to duplicate common facilities in each train, such as the dehydration vessels, acid gas removal vessels, and refrigerant compression and cryogenic liquefaction equipment. Accordingly, a continuing search has been directed to the development of systems and methods for reducing the unnecessary expense for these duplicate vessels.